Thanks for this HOWTO, I first had some issues, but after a clean install of the latest XAMPP and XAMPP Dev package and a clean download of PHP 5.3.0 it worked. I did have to manually add the line "extension=tidy.so" in php.ini.

I would just like to add, this fails on 5.3.1 on SnowLeopard. The tidy.so is created and added to the extensions folder, however it wasn't added to the php.ini and also when the server is restarted php gives a startup error.